The present invention relates to a concept for transmitting payload data (useful data, Nutzdaten) in the form of a plurality of coded data packets that may be combined, at the receiving end, such that they are adapted to a transmission quality, so as to adapt a coding gain to the transmission quality and/or to a transmission situation. Embodiments of the present invention may be employed, in particular, in unidirectional multipoint-to-point transmission systems.
For transmitting small payload data quantities as arise, e.g., with measuring devices, such as heating, electricity or water meters, for example, two different transmission modes may be employed, in principle. For one thing, sensor and/or payload data may be transmitted from a transmitter associated with the measuring device in question to a central receiver by means of unidirectional transmission (multipoint-to-point transmission). With such unidirectional transmission, the transmitter cyclically transmits its transmitter identification and a current sensor value at specific transmission times, which in most cases are selected randomly. Time lags between the transmission times are mostly adapted to a battery characteristic and selected such that a battery life becomes maximal. In this context, the transmitter will receive no confirmation whatsoever from the central receiver concerning the receipt of the sensor value, i.e. it has no knowledge of whether or not a transmission packet containing the sensor value has arrived at the receiver and/or has been able to be decoded. If, however, such an acknowledgement of receipt (ACK/NAK) is desired, one may fall back on bidirectional transmission.
In bidirectional transmission, a transceiver is provided at the sensor end. The transceiver transmits its sensor data and/or data packets only when asked to do so by a remote-side sensing device (central receiver). To this end, the sensor-end transceiver must constantly listen in to a radio channel to find out whether or not there is a transmission request directed at it.
For sensor and/or payload data transmission, so-called (wireless) sensor networks are also being used more and more frequently, wherein information about individual subscribers or nodes of the network are relayed until they eventually arrive at the desired information receiver. In this manner, data may be routed over a long distance if sensor nodes exist accordingly.
For sensor and/or payload data transmission, simple low-cost telemetry transceivers comprising amplitude (ASK) or frequency modulation (FSK) are mostly employed in the above-mentioned system approaches. In this context, reception is often not coherent, and in most cases no channel coding is utilized.
In contrast, in more complex digital wireless communication systems, transmission modes are used nowadays which transmit information and/or payload data such that they are distributed to different data packets that are sent out in a temporally and/or spatially offset manner, and such that they have different redundancy information, i.e. are channel-coded differently. Given high signal quality, i.e. a high signal-to-noise ratio (SNR), the coded data packets may be received and decoded individually. If the SNR at the receiver decreases, a code gain or coding gain may be realized by combining two or more data packets received. In coding theory, the code gain describes a difference of a required bit energy in relation to a noise power spectral density between an uncoded and a coded message in order to achieve an identical bit error rate. The uncoded message represents the reference with which the message coded by means of channel coding is compared. Such transmission modes, which are also referred to as code-combining and/or incremental-redundancy transmission modes, have been frequently applied, in conventional art, with so-called packet-oriented automatic repeat request protocols (ARQ protocols). If an error arises, at the receiving end, in decoding a data packet, a further data packet with redundancy, i.e. a further coded data packet, is requested at the transmitter via a return channel.
ARQ protocols are used in communication networks to guarantee reliable data transmission by means of repeat transmissions. By means of a possibility of error recognition, a receiver may ascertain any transmission errors that have occurred in data packets. Via the return channel, said receiver may communicate the result of the error recognition to the transmitter of the data packet. This is usually effected by transmitting so-called ACK/NAK signals (acknowledgement or negative acknowledgement, i.e. correct receipt confirmed, or request for repetition). If need be, a disturbed message is retransmitted until such time as it has reached the receiver without any errors.
So-called hybrid ARQ protocols (HARQ) represent an extended variant of the ARQ protocol, which comprises combining ARQ mechanisms, such as check sum formation, block confirmation and/or block repetition, with error-correcting coding. In this context, payload data may be channel-coded with an error-correcting block code or an error-correcting convolution code. I.e., unlike ARQ methods, wherein only error-recognizing redundancy information (e.g. CRC) is transmitted in addition to the payload data in a data packet, HARQ methods additionally comprise transmission of error-correcting redundancy information in the data packet in accordance with forward error correction methods (FEC methods). One may basically distinguish between three different types of HARQ methods:
The simplest version, type I HARQ, adds both error-recognizing and error-correcting redundancy information to the payload data prior to each transmission so as to obtain a coded data packet. When the coded data packet is received, the receiver first of all decodes the error-correcting channel code. Given sufficient transmission quality, all of the transmission errors should be correctable, and the receiver should thus be able to obtain the correct payload data. If the transmission quality is poor, and if, consequently, not all of the transmission errors can be corrected, the receiver may ascertain this by means of the error-recognizing code. In this case, the coded data packet received is discarded, and repeat transmission is requested via a return channel. Thus, type I HARQ designates transmission with perfectly identical repetition of the data sent in the initial transmission. Upon renewed reception of the data, information that was generated in the previous reception of said data may be reused. A possible principle for this is known from IEEE Transactions on Communications, Vol. COM-33, No. 5, May 1985, D. Chase, “Code Combining—A Maximum-Likelihood Decoding Approach for Combining an Arbitrary Number of Noisy Packets”. In this context, payload data is transmitted in data packets that are coded with a code having a relatively high code rate R and that are repeated to achieve reliable communication if the redundancy of the code is not sufficient to overcome, e.g., channel interference problems. The receiver combines received noisy data packets to obtain a combined data packet having a code rate R′<R small enough to ensure reliable combination even with transmission channels that cause extremely high error rates. In this context, one tries to reduce the delay (caused by packet repetitions) to a minimum by combining a minimum number of data packets while realizing a sufficiently good and high code rate in order to reliably decode the payload data transmitted.
In accordance with a further conventional method, logarithmic likelihood ratios (LLR—log likelihood ratios) for payload data of the previously transmitted data packet, said payload data having to be decoded, are determined during a decoding attempt of a previously transmitted data packet. If a decoding attempt fails, renewed transmission of the corresponding data packet is effected. For decoding the payload data of the newly sent data packet, the LLRs determined during the previous decoding attempt are utilized as a-priori information in a forward-moving procedure, similar to the known turbo-code principle.
With type II HARQ, a repeat transmission does not involve repeating precisely the data of the initial transmission, but involves transmitting additional redundancy that would not be decodable on its own without the data of the initial transmission (non-self-decodable). Such type II HARQ methods are typically also referred to as incremental-redundancy HARQ methods. In this context, the payload data and error-recognizing bits (CRC) are initially coded at the transmitter end, for example by means of a systematic “parent” code. This results in a code word consisting of systematic bits and so-called parity bits. In the first data packet sent, the systematic portion of the code word and a specific number, i.e. not all, of the parity bits which together form a code word of a parent code are sent. Said code word is coded at the receiver end. If this is not possible and if repeat transmission is requested, the transmitter will transmit, in a subsequent data packet, additional parity bits of possibly different powers and/or at altered channel conditions. Upon reception of the subsequent data packet, a new decoding attempt is made, which involves combining the additional parity bits with the ones previously received. This process can be repeated until such time as all of the parity bits of the parent code have been transmitted.
As was already described at the outset, there are simple digital wireless communication systems comprising only unidirectional transmission from the transmitter to the receiver, i.e. without any return channel. Such unidirectional multipoint-to-point communication systems are particularly suitable for low-cost transmission of small quantities of payload data as arise, e.g., with measuring devices, for example heating, electricity or water meters. However, with such communication systems, wherein a multitude of transmitters communicate with one receiver (multipoint-to-point), there is the problem that substantial interference may result at the receiver, depending on the number of transmitters and their random transmission times. Due to random transmission times of the transmitters and their number, which is often not predictable either, the interference or reception quality achieved at the receiver is not predictable. Nevertheless, it is to be ensured that transmitter-specific payload data can be decoded fast, efficiently and reliably at the central receiver even under most diverse receiving conditions.